A fuel cell stack is a power generation system which oxidizes a fuel by an electrochemical process to directly convert energy released by an oxidizing reaction into electric energy. The fuel cell stack has a membrane-electrode assembly in which both side surfaces of a polymer electrolytic film for selectively transporting hydrogen ions are sandwiched by a pair of electrodes formed of a porous material. Each of the pair of electrodes has a catalyst layer which contains, as a main component, carbon powder carrying a platinum-based metal catalyst and which comes in contact with the polymer electrolytic film, and a gas diffusion layer formed on the surface of the catalyst layer and having both air permeability and electron conductivity.
In this type of fuel cell system, when a cell operation is continued in an operation region where a cell voltage is an oxidization potential (about 0.7 V to 1.0 V), hydroxides are adsorbed on the surface of the platinum catalyst of the catalyst layer, whereby an activation overvoltage increases, and hence output characteristics lower sometimes. In view of such a situation, Japanese Patent Application Laid-Open No. 2005-346979 describes processing of stopping the supply of air to the fuel cell stack and forcibly lowering the output voltage of the fuel cell stack by a DC/DC converter to lower the cell voltage to a reduction potential (e.g., 0.6 V or less), thereby removing the hydroxides from the surface of the platinum catalyst to recover a catalyst activity. It is also described in the document that a surplus power generated by the catalyst activation processing is charged into a battery for auxiliary machines.    [Patent Document 1] Japanese Patent Application Laid-Open No. 2005-346979